Crankshafts are used in engines to transfer rotational energy to a vehicle transmission to provide motive power to the wheels. Bearings are provided in the engine to support the crankshaft and guide rotation of the crankshaft. Specifically, bearing journals coupled to bearings are typically provided between each rod journal in the crankshaft. However, the bearings supporting the crankshaft generate friction during rotation, thereby decreasing crankshaft efficiency. Consequently, engine efficiency and therefore engine fuel economy are decreased.
US 2014/0041618 discloses a crankshaft design for a four cylinder engine with reduced journal bearings. The inventors have recognized several drawbacks with the crankshaft design disclosed in US 2014/0130561 when applying the approach to an engine with fewer cylinders. For example, removing crankshaft bearings in an engine having 3 cylinders may increase bending of the crankshaft beyond a desirable level, thereby increasing engine vibration. Furthermore, engines with an odd number of cylinders, such as a 3 cylinder engines, typically have an even number of bearings and corresponding bearing journals. Therefore, removing a single bearing and bearing journal in the crankshaft makes the bearing arrangement non-symmetric with regard to a longitudinal direction. Non-symmetric distribution of the bearings and corresponding bearing journals unduly increases bending in the unsupported section which again leads to increased engine vibration and decreased crankshaft and bearing longevity.
As such in one approach, a crankshaft in an engine is provided. The crankshaft includes A crankshaft in an engine is provided. The crankshaft includes only two outer bearing journals configured to attach to two outer crankshaft bearings, only a single inner bearing journal positioned axially between the two outer bearing journals configured to attach to an inner crankshaft bearing, an unsupported section positioned axially between the inner bearing journal and one of the outer bearing journals, and only three rod journals each configured to attach to a separate piston rod. Providing a crankshaft with only three bearing journals and three rod journals enables the crankshaft friction to be reduced when compared to previous crankshafts for three cylinder engines that include four bearing journals and corresponding crankshaft bearings. As a result, fuel economy is improved. Additionally, using only three bearing journals enables the cost of the engine to be reduced, due to the elimination of a crankshaft bearing and corresponding parts. Furthermore, the mass of the engine can be reduced when a crankshaft bearing and corresponding parts are eliminated in the engine.
In one example, the unsupported section and the inner bearing journal are offset from a longitudinal centerline of the crankshaft. In this way, spacing of the cylinders in the engine can be maintained at desired distances when a bearing journal in the crankshaft is removed.
In one example, the unsupported section has a larger diameter than the bearing journals. As a result, crankshaft bending, torsion, and fatigue are reduced to provide desired crankshaft rotational characteristics. Specifically, the increased diameter of the unsupported section increases crankshaft stiffness in a desired area that may be particularly susceptible to bending.
Further in one example, two of the three rod journals are positioned axially between one of the outer bearing journals and the single inner bearing journal and one of the rod journals is positioned axially between the single inner bearing journal and the other outer bearing journal. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Additionally, the above issues have been recognized by the inventors herein, and are not admitted to be known.